Heterologous proteins capable of transducing optical stimuli into electrical signals can be used to control the function of excitable cells in intact tissues or organisms. Restricted genetically to circumscribed populations of cellular targets, these selectively addressable sources of depolarizing current can be used to supply distributed inputs to neural circuits, elucidate functional synaptic connections, probe the response characteristics of circuits and systems, and unveil behaviorally relevant information carried in distributed neural representations. To achieve genetically localized photostimulation, we express ligand-gated ion channels in neurons that normally lack them, and render the agonists that gate the conductances of these channels biologically inert by chemical modification with photoremovable blocking groups ('cages'). A key-and-lock mechanism thus ensures temporal control and cell-type specificity of photostimulation: the initiation of an action potential requires a light pulse that liberates free agonist (the 'key'), and a target neuron that has been genetically programmed to express the cognate ligand-gated ion channel (the 'lock'). The Objective of this project is to advance the development of highly selective phototriggers with fast kinetics (Specific Aims 1 and 2), create modular mammalian expression systems (Specific Aims 3 and 4), and initiate optical analyses of functional neural circuits in the mammalian brain, with an emphasis on networks of inhibitory (GABAergic) interneurons in the cortex and hippocampus (Specific Aim 5). GABAergic circuits are of basic importance to the processing, storage, and retrieval of information, as illustrated by the effects of agents such as ethanol and benzodiazepines, and by the involvement of interneurons in diseases such as schizophrenia and Alzheimer's disease.